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June 26, 2008

Dead Trees Tell Water Tales: What If Current Drought, Lake Levels Worsen?

by Sam Savage

By Alex Breitler, The Record, Stockton, Calif.

Jun. 26--Two years of drought have California's water managers scrambling.

What if we had 200 years of drought?

It's happened, a number of scientists say. And it could happen again.

Trees hidden beneath the waters of Sierra Nevada lakes suggest California, and most of the West, experienced "megadroughts" that put our current water crisis in context.

These Medieval dry spells -- if two centuries can be considered a "spell" -- dropped water levels so low that trees began growing as much as 70 feet below the current surface of one Sierra lake, researchers say.

When a wet climate returned, the trees drowned. But they remain rooted in the lake bottom; sometimes their ghostly tips still are visible when water levels drop in the fall.

"People have correlated tree ring thickness with things like stream flow," said Chuck Parrett, a hydrologist with the U.S. Geological Survey in Sacramento. "It's an estimation, but you're certainly right that some of the studies have indicated there have been really long dry periods in California."

Precipitation and stream flow records in California go back no more than 150 years, and the earliest are spotty at best. With only rough ideas of what the preceding centuries were like, Scott Stine, a geographer with California State University, East Bay, says our entire perception of California's climate may be off.

"What we have come to consider normal is profoundly wet," Stine told National Geographic magazine earlier this year. "We're kidding ourselves if we think that's going to continue, with or without global warming."

In published journals, Stine has described his work documenting the great droughts that took place before the arrival of Europeans.

As early as 1960, researchers discovered long-dead trees in Mono Lake east of Yosemite National Park that only recently had emerged as much of the lake's inflow was being diverted to Los Angeles.

Stine later found dozens more stumps -- Jeffrey pines, cottonwoods and shrubs -- some of which died about 1100 AD, and others of which died about 1350 AD.

Years of follow-up research led him to conclude that a severe "double-drought" had taken place.

He found a dozen large tree trunks poking out of Tenaya Lake in Yosemite National Park and came up with similar dates for that wood.

Even more relics were found in the West Walker River and Owens Lake in the eastern Sierra.

The cause of droughts in California has not changed, Stine said. The storm track that drenches California during wet winters was apparently anchored over Alaska, where glaciers grew even as California withered in drought.

This past century, it turns out, has been abnormally wet.

"Drier times undoubtedly lie ahead," Stine wrote.

Similar studies have documented oppressive droughts in the Colorado River basin. Indeed, when the river water was allocated to farms and cities in the early 1920s, it may have been based on an overly optimistic idea of what a truly normal wet season would provide.

While many plants and animals evidently survived these dry spells, some speculate that megadroughts throughout the West may have led to the abandonment of American Indian cliff dwellers, who had relied on irrigated agriculture.

In contemporary times, climatologists consider the drought of 1929-34 to be the worst on record, though 1977 was the single-driest year.

Tree ring records give a "fairly good measure" of what conditions must have been like over the thousands of years for which we have no records, said Maury Roos, a hydrologist with the state Department of Water Resources.

They are not perfect, he said.

While climate change may contribute to more frequent drought in the future, it's also likely we'll see more storms and precipitation in those years that are wet, Roos said.